Questions involving using VBA to do some advanced task in Outlook or Word seem like they belong on Super User, but I'm not entirely sure. I think questions like this one straddle the line between the two sites, and could possibly be asked on both. What does everyone else think? Should questions like this be migrated to one site or the other, or just left where they're originally posted?
I believe it is a judgement call based on each question. I have passed a few VBA question on SU, which based on the way it was asked indicates the user is simply a power user and needs gentle guidance, compared to the question in your example that shows some throrough research on the topic. This is what I use as my guide. If the question requires developer level understanding --> SO, if not --> SU.
Based on the level and quality of the question, leave it where it was posted, unless it will get a better response somewhere else.
Interesting question, but I think a good rule of thumb would be based upon if what you are trying to accomplish could be recorded or not. If you have to actually hand write the macro then I would say it belongs on Stack Overflow. However, if you are able to just use the macro recorder and go through a series of menu commands then I would say that it belongs on Super User.
This is a good example of a question that belongs on both sites. See here: Allow cross-posting of questions to more than one SO site